In classical arthroplasty alignment, the distal femoral cut is perpendicular to the femoral mechanical axis. This axis is defined as a line drawn through the center of the femoral head to the center of the knee, which is defined by the center of the intercondylar notch, with the patient standing and weight bearing. Under current practice, most surgeons prefer a zero degree cut; that is, a perpendicular cut on the tibia. This is a line from the center of the tibial anatomy, which is at the center of the tibial spines, to the center of the talus. The goal here is a right angle cut to that alignment axis. When these two landmarks are combined, the result should be a well-balanced knee in the coronal plane, meaning with the knee in full extension at 30 and 60 degrees of flexion. There should also be minimal to trace laxity in that knee on examination at the time of the arthroplasty procedure.
Historically, if the alignment is not appropriate and there is imbalance present after the bone cuts have been made, most surgeons assume that there arc problems with the soft tissue balancing and that there are contractures of the medial or lateral structures which are preventing perfect balancing of the knee. While this may be true in a small portion of knees with significant combined sagittal and coronal deformity, I believe that improper bone cuts are the problem and not ligament balancing. I believe this is true in 95 percent of total knee replacement cases.
Currently, most surgeons use instrumentation which has been around for approximately 40 years. Using these instruments on the femoral side, surgeons first position an intramedullary rod, which is inserted through a hole drilled in the distal femur. After the rod is placed, a five degree valgus alignment for the cuts on the distal femur is arbitrarily and traditionally chosen for most male patients. This five degree valgus alignment is chosen because typically in men the difference between the anatomic axis, which is the line drawn up the femoral shaft to the center of the femur, and the mechanical axis, which is the line drawn from the center of the femoral head to the center of the femur is roughly five degrees. This difference in women it is approximately seven degrees. Thus, when using the intramedullary alignment on the femur, this angle is chosen for the cut on the distal femur to achieve proper alignment.
However, as learned from computer navigation, these historically elected angles may be inappropriate most of the time. Some patients may exhibit 3.8 degrees of valgus in their alignment, others may be 7.3. The fact is, true accuracy can never be realized with an educated guess. Computer navigation is perhaps the most accurate way to make this cut. Navigated procedures actually locate the center of the femoral head based on complex mathematic algorithms, and “registration points” on the bones taken at the time of surgery, using existing, identifiable landmarks. However, computer navigation is expensive, it is time consuming, and it will probably not ever be available to every surgeon.